Adhesive and method of applying same

ABSTRACT

An adhesive is provided which includes polyvinyl butyral to improve the corona starting voltage characteristics of the adhesive. The adhesive is a mixture of a polyvinyl butyral varnish with an equal amount of phenolic resin varnish. The phenolic is a predominant ingredient by weight in the phenolic resin varnish but the polyvinyl butyral is not the predominant ingredient in the polyvinyl butyral varnish so that the resultant adhesive, although containing polyvinyl butyral, is predominantly phenolic.

United States Patent 2,745,073 5/1956 Swidelsky 336/60 2,870,295 1/1959Haroldson 6131.. 156/335 X 2,917,481 12/1959 Masters 260/844 2,964,49112/1960 Rylander et al. 260/844 3,056,984 10/1962 Snitzer 12/1423,141,005 7/1964 Nullum. 260/844 X 3,435,127 3/1969 Rose 156/305 X3,442,834 5/1969 Flowers et al 260/844 Primary Examiner-Harold AnsherAssistant Examiner-D. J. Fritsch Artorneys-John W. Michael, Gerrit D.Foster, Bayard H.

Michael, Paul R. Puerner, Joseph A. Gemignani, Andrew 0. Riteris andDaniel Van Dyke ABSTRACT: An adhesive is provided which includes polyvi'nyl butyral to improve the corona starting voltage charac teristics ofthe adhesive. The adhesive is a mixture of a polyvinyl butyral varnishwith an equal amount of phenolic resin varnish. The phenolic is apredominant ingredient by Weight in the phenolic resin varnish but thepolyvinyl butyral is not the predominant ingredient in the polyvinylbutyral varnish so that the resultant adhesive, although containingpolyvinyl butyral, is predominantly phenolic.

PATENTEUNUVE 197i 3.617.969

' SHEET 1 Hi2 I @WMM d/ttarr/d1 ADHESIVE AND METHOD OF APPLYING SAMEThis application is a continuation-in-part of my prior field copendingapplication Ser. No. 601,292 filed Nov. 21, 1966 and entitled Adhesiveand Method of Applying Same," and assigned to the assignee of thisapplication.

BACKGROUND OF INVENTION This invention relates to adhesives for use inelectrical apparatus and to the method of applying such adhesives. Moreparticularly, the inventionrelates to adhesives as may be used to joinelements which, in operation, are exposed .to a highstrength electricalfield, the adhesive of this invention being effective to increase thecorona starting voltage in that field.

The occurrence of corona discharge in various electrical applications isa well-known phenomenon. Also, it is well recognizedthat coronadischargecan adversely affect the operation of various electricalapparatus and, also, can deteriorate, the electrical apparatus itself.It has been discovered that, in electrical apparatus, the adhesive usedto join elements which are disposed in a relatively high-strengthelectrical field has a marked effect on corona starting voltage in thatfield. I-Ieretofore, in providing adhesives to join elements inelectrical apparatus consideration has only been directed tothedielectric strength of the adhesive and its compatibility with othermaterials and insulation used in the electrical apparatus. Generally ithas not been recognized thsttheadhesive has any effect on the coronastarting voltage and, cor sequently, adhesives have generally beenprovided without regard to any effect the adhesive interface might haveon the corona starting voltage.

For example, in a power transformer, adhesives are used at ,variouspoints to connect elements such as insulation members provided in thetransformer coil and core. The adhesive interface so formed is disposedin a relatively high-strength electrical field and has a marked effecton the corona starting voltage or gradient. A low corona startingvoltage can result in corona discharge at an undesirable low level andsuch discharge, among other detrimental effects, has a tendency. todegrade the electrical insulation within the power transformer and leadto electrical breakdown. i

SUMMARY OF INVENTION This invention arises out of the discovery that theadhesive has a marked effect on corona starting voltage and can providea significant increase in the corona starting voltage so as to precludecorona discharge under normal operating conditions for the particularelectrical apparatus,

Accordingly, an object of this invention is toprovide a new and improvedadhesive which significantly increases the corona starting voltage in arelatively high-strength electrical field.

Another object of this invention is to provide an adhesive interfacewhich substantially increases corona starting voltage and a method ofapplying the adhesive to achieve that interface.

For the achievement of these and other objects, this invention proposesan adhesive for joining members in a relatively high voltage gradientelectrical field, which adhesive is a mixture of polyvinyl butyral and aphenolic, with the phenolic, with respect to said polyvinyl butyral,being the predominant ingredient in the adhesive. This results in anadhesive which exhibits improved corona starting voltage characteristicsdue to the polyvinyl butyral and also has satisfactory mechanicalproperties due to the phenolic.

The adhesive is provided by mixing an amount of polyvinyl butyralsolution with an equal amount of phenolic resin solution. The phenolicis the predominant ingredient by weight in the phenolic resin solutionwhereas the polyvinyl butyral, by weight, is not the predominantingredient in the polyvinyl butyral solution. Thus, the resultingadhesive contains polyvinyl butyral for improving corona characteristicsbut it is predominantly phenolic. x

DESCRIPTION OF DRAWINGS DESCRIPTION OF PREFERRED EMBODIMENT An adhesiveformulated in accordance'with this invention comprises a mixture ofpolyvinyl butyral with a conventional phenolic. It has been discoveredthat polyvinyl butyral exhibits extremely good (i.e. high) coronastarting voltage characteristics so that when mixed with phenolicresins, which generally exhibit relatively poor (i.e. low) coronastarting voltage characteristics but desirable mechanical properties,the

resulting adhesive will exhibit a significantly improved corona istarting voltage.

For convenience, the adhesive of this invention will be discussed inconnection with a power transformer, but it is not necessarily limitedto any particular application.

Heretoforeconventional adhesives used in electrical apparatus such aspower transformers have been phenolic, polyvinyl alcohol, epoxy, shellacor similar type adhesives. These adhesives were selected for theirinsulating characteristics. and also for their compatibility with othermaterial and insulating media used in the electrical apparatus, such asoil orgas. Polyvinyl butyral, as such, has been used in adhesives in thepast but has never been considered from the standpoint of any effect itmight have on corona starting voltage.

For example, polyvinyl butyral has been considered for its adhesiveproperties in connection with joining various synthetic elements tometal and also for its mechanical characteristics as an effectiveadhesive, for example polyvinyl butyral is recognized as being capableof exhibiting relatively high ultimate shear strength. However,heretofore no one has recognized the significant electricalcharacteristic of polyvinyl butyral, namely its ability to substantiallyincrease corona starting voltage, nor has it ever been recognized thatthe adhesive used could affect corona starting voltage.

The adhesive contemplated by this invention is a combination ofpolyvinyl butyral and phenolic, but is predominantly phenolic. Anexample of a formulation which results in an adhesive having both theimproved mechanical characteristics and the improved corona startingvoltage characteristics of this invention is a mixture of two solutions(I) a phenolic resin varnish (for example that available under thecommercial designation 358 Sterling 358 which is a conventionalphenolaldehyde which consists of 54.5 percent (by weight) phenolic resinin denatured alcohol and (II) a polyvinyl butyral varnish which consistsof 16.7 percent (by weight) polyvinyl butyral in denatured alcohol.These two solutions are mixed in equal proportions by weight or volume,50 parts of the phenolic resin varnish with 50 parts of the polyvinylbutyral varnish. The resulting adhesive is predominantly phenolic and asa result exhibits good thermosetting characteristics and will providehigh bond strength throughout the normal range of temperaturesencountered in a power transformer. Moreover, with the inclusion of thepolyvinyl butyral the corona starting voltage gradient of the resultingadhesive is markedly improved, being in excess of 160 kv. per inch. Thiscorona starting gradient represents an increase of approximately percentover that exhibited by a phenolic adhesive alone. For example, thephenolic resin adhesive, Sterling Y-358, alone normally exhibits acorona starting voltage gradient of 80 kv. per inch. Furthermore, thiscorona starting voltage of kv. per inch is a significant improvementover other commonly accepted adhesives such as resin shellac whichprovides a corona starting voltage of approximately 30-50 kv. per inch.

FIG. 6 is a curve plotting corona starting voltage against differentpercentages by weight of polyvinyl butyral in the adhesive mixture ofpolyvinyl butyral varnish and phenolic resin varnish. The extreme leftof the curve (polyvinyl butyral) illusto be connected. The layer is thensmoothed, with the use of a doctor blade, scraper or the like, to reducethe thickness of the layer while at the same time breaking up any voidsand pockets to release any entrapped medium. The adhesive coatagainresults in a market improvement in corona starting voltage.

' It has also been discovered that the corona starting voltage can beimproved still further by providing an adhesive coating which isrelatively thin and of uniform consistency i.e.. free of 70 voids orpockets containing entrapped air, other gaseous media or solution (inthis instance the alcohol). To achieve an adhesive layer having thesecharacteristics this invention proposes coating the phenolic-polyvinylbutylral fluid adhesive mixture in a normal manner onto ohe or bot o theelements "ates the corona StartihB Voitage With P Phehoiic resin ing isthen baked for approximately 5 minutes at approximatesolution orvarnish. The extreme right of the curve illustrates l 200 F Thi baking,or B-staging, removes approximately the corona Starting Whale for PPoiyvinyi butyral sohitioh 90 percent of the solvent, again it beingnoted that the en- Or Varnish aplii'oiiiihhteiy Percent by Weight oftrapped medium can be bubbles of either air, gas, or the alpolyvinylbutyral in the adhesive). The following table is cohol of the solution.Preferably the thickness of the adhesive related with the curve of 6 andgives the various values layer is controlled and does not exceed 0.003inches when for different mixtures of polyvinyl butyral varnish andphenoldriecp ic resin varnish but all mixtures providing a predominatelyphenolic adhesive. The 100-100 pound mixture of polyvinyl The adhesive yis how ready to he used eohheetihg butyral varnish and phenolic resinvarnish corresponds to the two eiemehts- This can be accomplished yheatactivatihg the 50-50 part mixture described above. It should benoted, how is adhesive layer and bringing the eiemehts ihto Contactthrough ever, h h corona Starting voltage i h bl i given i the adhesivelayer. The heat-activated adhesive forms a tack absolute values whereasin the previously described example it bond between the elements and theadhesive layer is cured to was expressed as a gradient. set the adhesiveand produce the final bond, for example the Ratio 312225: Corona 2 61 152 Total Mixture Mixture h varnish starting PVB Phenolic PVB weightweight weight Percent resin to Y-368 voltage varnish resin resin Solventmixture percent percent solids phenolic (lbs.) (kv.) Obs.) (lbs.) (lbs.)(lbs.) (lbs.) phenolic PVB mixture resin 100 57 s0 s2 13. 4 114. 6 18028. 7. 36. 4 25. 8/100 100 58 90 52 15.1 122. 9 190 27. 4 7. 9 35.29/100 100 60 100 52 16. 7 131. 3 200 2a 0 s 1 34. 4 32/100 100 00 12052 20. 0 148. 0 220 24. 6 9. 1 3a. 7 38. 5/100 100 60 140 52 23. a 164.7 240 21. 0 9. 7 31. 3 45/100 Referring to FIG. 6, a satisfactoryadhesive from both a layer is cured by subjecting it to a temperature ofapproximechanical and electrical standpoint is one wherein the I mately135 C. for approximately 12 hours. amount of polyvinyl butyral in thestarting adhesive solution is An alternative method of softening theadhesive film after in the range of about 6 to about 10.5 percent byweight based B-staging is through the use ofa suitable activator. Atypical on he o l W igh f h l ion- Optimum results are activator whichhas given satisfactory results is a mixture of tained when the amount ofpolyvinyl butyral in the starting 135 denatured alcohol and acetone,preferably nine parts alcohol Soiutioh is in the range about aboutPercent y to one part acetone. After softening, or activation, of thefilm Weight based on the totai weight of the sohitioh, the P of occursthe pieces to be bonded are brought together with light the tii'st riseof the Tahiti i shows the weight ratio pressure. This again produces aninitial tack bond as the acpolyvinyl butyral to the phenolic resin inthe resultant adhe- 4O tivating medium dissolves into the adhesiveparts. After the SM: composition will h in a range equivalent to about htack bond a final cure is made and air, gas and/or solution are aboutlbs. of polyvinyl butyral per 100 lbs. of the phenolic driven off bybaking the elements and adhesive layer an resin proximately 135 C. for12 hours.

With particular reference to the drawings, FIGS. 1 and 2 il- Thepolyvinyl butyral and phenolic mixture adhesive prolustrate aninsulating barrier I normally used to separate the 45 vides a strongmechanical bond between the elements, is coil assemblies in a shell-formtransformer. In this application e e all m ible ith the va i i l ti edid i a n m of sp blocks WhiCh can be of any Suitable electrical apparatusand, in addition, markedly increases the electrical insulating materialsuch as Kraft board, are attached corona Starting voltage across theadhesive im f For to a panel 3, again of any suitable electricalinsulating materiample within the normal operating range f a powertrans- Gehei'aiiy, the Connection between the spacer hietfks and formerthe occurrence of corona in the areas influenced by the Pahei is matiegh h e y it is t t i Poilit the adhesive is virtually precluded. Thisbeing particularly true that the adhesive f this ihvehhoh "3 By loimhg hwhere further attention is directed to the application and blocksahdiahei using a butyral phenolic i treatment of the adhesive to insurea layer which is free of adheslYe the corona p is markedly voids and anyentrapped medium and is also relatively thin creased as discussed abovei and uniform. This adhesive can be used effectively to join any FIG. 3,4 and 5 illustrate a portion ofa coil assembly 5 used elements which areto be disposed in a high electrical field and in core-form transformer,one alternative application of the where corona discharge may be aproblem. adhesive of this invention. windings 6 are separated at spacedAlthough this invention has beenillustrated and described points byradial spacers 7 of insulating material. The radial in connection withparticular embodiments thereof, it will be spacers are connected to acylindrical barrier 8 which is ofina e t [h e kill d i th n th t b i h ed sulating material and surrounds a leg of the transformer core difi imay b d h i i h departing f o (not shown). The connection of the radialspacers to barrier 8 h i it f h i i ism s si slzis 9 fis hs.mhs b r rbxq s iv lclaimz layer 10. The adhesive of this invention is used at layer10 and 1, I bi i fi d second members d d to b disposed in a relativelyhigh voltage gradient electrical field, and an adhesive interfacebetween and joining said first and second members and also adapted to bedisposed in said field, said adhesive consisting essentially ofpolyvinyl butyral and a phenol-aldehyde resin in amounts equivalent tothe ratio of about 25.8 to about 45 pounds of said polyvinyl butyral perpounds of said phenol-aldehyde resin so that said adhesive has theeffect of substantially increasing the corona starting voltage in saidfieldin addition to affording a connection between said first and secondmembers.

2. The combination of claim 1 wherein the thickness of said adhesiveinterface is of substantially uniform thickness and is substantiallyfree of voids and media other than said polyvinyl butyral andphenol'aldehyde.

3. The combination of claim 2 wherein the thickness of said adhesiveinterface does not exceed 0.003 inches.

4. The combination of claim 1 wherein said first and second members arepart of a transformer assembly said transformer and assembly comprising,

a coil and core assembly,

said first and second members comprising electrical insulating membersmaking up a part of said coil and core assembly and positioned to be inthe electrical field of said coil and core assembly when said coil isenergized.

5. The process comprising the steps of preparing an adhesive mixture byadmixing a polyvinyl butyral solution with a phenol-aldehyde resinsolution, the solids in the resultant adhesive mixture beingpredominantly said phenol-aldehyde with the amount of said polyvinylbutyral in said resultant adhesive mixture being in the range of about6.5 to about 10.5 percent by weight based on the total weight of saidresultant adhesive mixture solution,

applying said resultant adhesive mixture between two members to bejoined,

and setting said resultant adhesive mixture to provide an adhesiveinterface between and joining said two members, whereby when said joinedmembers are disposed in a relatively high voltage gradient electricalfield, said adhesive interface has the effect of substantiallyincreasing the corona starting voltage in said field in addition toafiording the connection between said first and second members.

6. The process of claim 5 wherein the solutions mixed to prepare saidadhesive comprise polyvinyl butyral in a denatured alcohol solution andphenol-aldehyde resin in a denatured alcohol solution,

and including the step of heating said adhesive mixture afterapplication to release entrapped media.

7. The process of claim 5 wherein said adhesive mixture is applied toone of said members,

and said members are brought together with said adhesive mixturetherebetween and said adhesive mixture is treated prior to bringing saidmembers together to thereby soften said adhesive to produce a tack bondbetween said members.

8. The process of claim 5 wherein a quantity of said polyvinyl butyralsolution including approximately 16.7 percent by weight ofpolyvinylbutyral is admixed with an equal quantity by weight of saidphenolaldehyde resin solution which includes approximately 54.5 percentby weight of phenol-aldehyde resin.

9. The process of claim 5 wherein the amount of said polyvinyl butyralin said resultant adhesive mixture is in the range of about 8.5 to about9.5 percent by weight based on the total weight of said resultantadhesive mixture.

10. The process of claim 5 wherein the amount of said polyvinyl butyraland said phenol-aldehyde in said resultant adhesive mixture isequivalent to the ratio of about 25.8 to about 45 lbs. of said polyvinylbutyral per lbs. of said phenol-aldehyde resin.

i ll i 0

1. In combination, first and second members adapted to be disposed in arelatively high voltage gradient electrical field, and an adhesiveinterface between and joining said first and second members and alsoadapted to be disposed in said field, said adhesive consistingessentially of polyvinyl butyral and a phenol-aldehyde resin in amountsequivalent to the ratio of about 25.8 to about 45 pounds of saidpolyvinyl butyral per 100 pounds of said phenol-aldehyde resin so thatsaid adhesive has the effect of substantially increasing the coronastarting voltage in said field in addition to affording a connectionbetween said first and second members.
 2. The combination of claim 1wherein the thickness of said adhesive interface is of substantiallyuniform thickness and is substantially free of voids and media otherthan said polyvinyl butyral and phenol-aldehyde.
 3. The combination ofclaim 2 wherein the thickness of said adhesive interface does not exceed0.003 inches.
 4. The combination of claim 1 wherein said first andsecond members are part of a transformer assembly said transformer andassembly comprising, a coil and core assembly, said first and secondmembers comprising electrical insulating members making up a part ofsaid coil and core assembly and positioned to be in the electrical fieldof said coil and core assembly when said coil is energized.
 5. Theprocess comprising the steps of preparing an adhesive mixture byadmixing a polyvinyl butyral solution with a phenol-aldehyde resinsolution, the solids in the resultant adhesive mixture beingpredominantly said phenol-aldehyde with the amount of said polyvinylbutyral in said resultant adhesive mixture being in the range of about6.5 to about 10.5 percent by weight based on the total weight of saidresultant adhesive mixture solution, applying said resultant adhesivemixture between two members to be joined, and setting said resultantadhesive mixture to provide an adhesive interface between and joiningsaid two members, whereby when said joined members are disposed in arelatively high voltage gradient electrical field, said adhesiveinterface has the effect of substantially increasing the corona startingvoltage in said field in addition to affording the connection betweensaid first and second members.
 6. The process of claim 5 wherein thesolutions mixed to prepare said adhesive comprise polyvinyl butyral in adenatured alcohol solution and phenol-aldehyde resin in a denaturedalcohol solution, and including the step of heating said adhesivemixture after application to release entrapped media.
 7. The process ofclaim 5 wherein said adhesive mixture is applied to one of said members,and said members are brought together with said adhesive mixturetherebetween and said adhesive mixture is treated prior to bringing saidmembers together to thereby soften said adhesive to produce a tack bondbetween said members.
 8. The process of claim 5 wherein a quantity ofsaid polyvinyl butyral solution including approximately 16.7 percent byweight of polyvinyl butyral is admixed with an equal quantity by weightof said phenol-aldehyde resin solution which includes approximately 54.5percent by weight of phenol-aldehyde resin.
 9. The process of claim 5wherein the amount of said polyvinyl butyral in said resultant adhesivemixture is in the range of about 8.5 to about 9.5 percent by weightbased on the total weight of said resultant adhesive mixture.
 10. Theprocess of claim 5 wherein the amount of said polyvinyl butyral and saidphenol-aldehyde in said resultant adhesive mixture is equIvalent to theratio of about 25.8 to about 45 lbs. of said polyvinyl butyral per 100lbs. of said phenol-aldehyde resin.